Titanium nitride (TiN) has been widely used for wear-resistant coatings, e.g. on cutting tools, for many years. More recently, a mixed titanium aluminum nitride has provided improved performance on drills, and at least one group has reported superior hardnesses and wear life using deposited coatings of titanium boronitride (Ti--B--N).
The methods described to date for depositing titanium boronitride coatings are dual target magnetron sputtering processes, in which two targets are sputtered with an argon-nitrogen plasma. The two sputter targets are (1) titanium, and (2) titanium diboride. The properties of the resulting titanium boronitride coatings depend strongly upon the chemical composition of the coating formed. The best coating properties are obtained when the coating comprises particles having a small grain size, because this imparts hardness, and when the coating has a boron content of about 20 atomic % or less in relation to the titanium content, and a nitrogen content of about 40 atomic % or less in relation to the titanium content. The Vickers hardness number reported for the titanium boronitride coatings formed using dual target magnetron sputtering was 6800 HVN. The abrasive wear resistance reported for the same coatings was nine times better than that of TiN.
Unfortunately, magnetron sputtering involves so many process variables that it is difficult to exercise a great degree of control over the chemical composition of the resulting titanium boronitride coating. The process variables which may affect the chemical composition of a coating produced by dual target magnetron sputtering include: (1) the position of the workpiece relative to the two sputter targets; (2) the argon flow rate; (3) the nitrogen flow rate; (4) the power deposited in the plasma discharge; and, (5) the bias voltage used. In addition to the large number of process variables that can affect the chemical composition of the resulting coating, dual target magnetron sputtering suffers from the disadvantage that it cannot readily be used to coat large components because the distance between the sputter targets and the workpiece cannot be greater than about twelve inches.
A process is needed which can be used to coat large scale components, and which provides more control over the chemical composition of the resulting titanium boronitride coating.